In today's rapidly advancing semiconductor manufacturing industry, the demand for increasing levels of device integration requires that device features are made increasingly smaller and in closer proximity to one another. The most critical steps in defining and ultimately producing device features are the photolithography and etching operations. As such, higher levels of device integration will likely be enabled by technological advances in the photolithography and/or etch processes. In order to meet this demand, processes for increasing photomask resolution, such as the process of optical proximity correction (OPC), are put forward constantly.
The object of OPC is to eliminate the phenomenon of the proximity effect in photolithography. In metal-oxide-semiconductor (MOS) devices, each of the several component layers, i.e., film layers and implant levels, is patterned using a photolithography step. Photolithography entails coating a substrate, such as a semiconductor wafer, with a photosensitive film commonly called photoresist, then exposing the photosensitive film by projecting light through a photomask that includes transparent areas and an opaque pattern. The photomask pattern is transferred to the photoresist layer producing a photoresist pattern which acts as a mask for subsequent doping or etching procedures.
A light beam that travels along the edge of an opaque feature produces a scattering phenomenon that enlarges the light beam and produces a scattering effect that distorts the pattern being formed. When the light beam passes through the photoresist layer on the substrate, it also reflects off the substructure beneath the photoresist layer and the phenomenon of interference results. As such, various phenomenon influence the projection of an opaque pattern from a photomask onto a photoresist layer. The smaller the critical dimension of the pattern features are, the more prominent these phenomenon become, especially when the critical dimension approaches half of the wavelength of the light source for exposure.
These exposure phenomenon combine to create the proximity effect which causes problems when densely packed features such as tightly packed parallel lines or intersecting lines undergo exposure at the same time. A corner formed of orthogonally intersecting lines in a mask pattern frequently produces an undesirably rounded structure when transferred to the device layer due to the proximity effect which causes light to scatter from the proximate orthogonal edges of the mask pattern. The proximity effect is greater closer to the intersection of the lines and causes rounding of the corner intersection, even though distal portions of the intersecting lines may be printed substantially straight.
The use of scattering bars represents one OPC technique used to correct and reduce the proximity effect in the photolithography process and to correct for mask bias differences between nested and isolated straight lines. This technique is not available to compensate for the proximity effect when substantially orthogonal lines intersect and the proximity effect increases toward the intersection.
It would therefore be desirable to eliminate the proximity effect at the intersection of orthogonal lines and produce device features with orthogonal corners that are accurately formed and do not exhibit rounding effects.